Nuclear reactor



Feb. 15, 1%6 M. F. SANKOVICH 3,235,463

NUCLEAR REACTOR Original Filed Jan. 31, 1958 10 Sheets-Sheet 1 VENTPRlMARY- RELIEF VALVE STEAM GENERATOR PRESSURIZER REACTOR PRIMARYCOOLANT PUMPS CONTROL L..R PURIFICATION RODS SYSTEM VALVE INVENTOR.

Melvin F. Sankovich AT TORN EY Feb. 15, 1966 Original Filed Jan. 31,1958 M. F. SAN KOVICH NUCLEAR REACTOR FIG. 2

10 Sheets-Sheet 2 INVENTOR.

Melvin F. Sankovich ATTORNEY Feb. 15, 1966 M. F. SANKOVICH 3,

NUCLEAR REACTOR Original Filed Jan. 31, 1958 10 Sheets-Sheet 5 M I IINVENTOR.

Melvin F. Sankovich BY WW ATTORNEY Feb. 15, 1966 M. F. SANKOVICH3,235,463

NUCLEAR REACTOR Original Filed Jan. 51, 1958 10 Sheets-Sheet 4 INVENTOR.Melvin E Sankovich ATTORNEY Feb. 15, 1966 M. F. SANKOVICH NUCLEARREACTOR Original Filed Jan 51, 1958 10 Sheets-Sheet 5 o o o c o o o onELM. QMRFEFQ m M T i o o o u 0 on m T m V m Melvin E Sankovich ATTORNEYFeb. 15, 1966 M. F. SANKOVICH 3,235,463

NUCLEAR REACTOR Original Filed Jan. 31, 1958 10 Sheets-Sheet 6 FIGS "-HIHH INVENTOR.

Melvin F. Sankovich ATTORNEY Feb. 15, 1966 M. F. sANKovlcH 3,

NUCLEAR REACTOR Original Filed Jan. 31, 1958 10 Sheets-Sheet '7 MIN lfll

A @2 WQi IHLJ llil' l lu INVENTOR.

Melvln F. Sankovich AT'T ORNEY Feb. 15, 1966 M. F. SANKOVICH 3, 3

NUCLEAR REACTOR Original Filed Jan. 31, 1958 10 SheetsSheet 8 FIG.1O

INVENTOR.

Mel vi n F. Sankovich fimw ATTORNEY Feb. 15, 1966 M. F. SANKOVICH 3,

NUCLEAR REACTOR Original Filed Jan. 31, 1958 10 Shets-Sheet 9 FIG. 11

INVENTOR Melvin E Sankovich BY fihw ATTORNEY Feb. 15, 1966 M. F.SANKOVICH 3,235,463

NUCLEAR REACTOR Original Filed Jan. 31, 1958 10 Sheets-Sheet 10INVENTOR.

Melvin F. Sankovich BY wan/W ATTORNEY United States Patent 3,235,463NUCLEAR REACTOR Melvin F. Sankovich, Lynchburg, Va., assignor to TheBabcock & Wilcox Company, New York, N.Y., a corporation of New JerseyContinuation of application Ser. No. 712,512, Jan. 31, 1958. Thisapplication Oct. 9, 1961, Ser. No. 145,012 4 Claims. (Cl. 176-17) Thisapplication is a continuation of my earlier filed co-pending applicationSerial No. 712,512, filed January 31, 1958, and now abandoned.

This invention relates in general to nuclear reactors wherein acontrolled fission chain reaction takes place, and more particularly itrelates to an internal converter type nuclear reactor wherein somefissionable material is created by the conversion of a fertile materialin the presence of a neutron flux.

In a nuclear reactor a neutron fissionable isotope such as U233, U andPu or mixtures thereof is fissioned by absorption of neutrons and aself-sustaining chain reaction may be established by the neutronsevolved from the fission if the mass of fissionable material is madesulficiently large. Specific details of the theory and essentialcharacteristic of such reactors are set forth in Patent No. 2,708,656,issued to Enrico Fermi et al. on May 17, 1955.

An internal converter reactor, as used in this invention, is one whereina fertile material contained within the reactor core is converted to afissionable material by exposure to reactor generated neutrons where thequantity of fissionable material converted is less than the fissionablematerial which is consumed during a given period of operation of afission chain reaction. An example of a fertile material which isconvertible to a fissionable material is thorium which upon neutroncapture is ultimately transformed to U Specific details andcharacteristics of this transformation are set forth in Patent No.2,798,- 847, issued to Enrico Fermi et al. on July 9, 1957.

Accordingly the present invention provides a nuclear reactor containinga supercritical mass of fuel containing a mixture of fertile andfissionable material disposed as a number of elongated andlongitudinally contiguous fuel element assemblies of heterogeneous for-mgeometrically arranged in a core to undergo a controlled chain typefission reaction. The reactor is formed by a vertically elongatedpressure vessel of circular cross section. An upper and a lower gridplate are arranged within the pressure vessel transversely ofits'longitudinal centerline. The fuel element assemblies are separate,replaceable and individually removable and have end portions arranged tobe fitted into the upper and lower grid plates. The upper grid plate isarranged to hold said fuel element assemblies in position irrespectiveof thermal expansion changes. Variable orifices are arranged in thelower grid plate to adjustably control the fiow of a cooling fluidtherethrough. The controls for the chain reaction are bottom mounted andvertically arranged to pass through the pressure vessel. Thermal shieldsare concentrically arranged about the core within the vessel and a lightwater filled neutron shield tank is arranged exteriorly about thepressure vessel.

Further, the present invention provides a multiplicity of controlrod-fuel element assembly sets which form the core. Each of the setscomprising a plurality of separate and individually removable fuelelement assemblies is symmetrically arranged to form therebetween anopen 3,235,463 Patented Feb. 15, 1966 ended centrally located controlrod channel throughout the length of the assemblies. Control rods havinga high neutron absorption cross section are arranged to belongitudinally positionable within the control rod channels. Controlmeans are provided to position the control rods longitudinally of thecore to maintain the controlled chain reaction.

Moreover this invention provides an arrangement whereby the fuel elementassemblies making up the core are arranged in a number of symmetricallyarranged longitudinally extending fuel concentration zones. While thefissionable to fertile material weight percent ratio is uniform in thefuel element assemblies within a zone, it is substantially differentfrom zone to zone. However, the fissionable material in the fuel elementassemblies throughout the core is enriched to substantially the sameextent irrespective of the variation in the fissionable to fertilematerial weight percent ratio.

Additionally, the invention provides that each fuel element assembly haswalls which are composed of a low thermal neutron absorption materialand which form a flow chamber containing a large number of spaced fuelcontaining means.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which a certain specific embodiment of the invention isillustrated and described.

Of the drawings:

FIG. 1 is a schematic diagram showing the reactor of the inventionwithin a heat transfer system;

FIG. 2 is a vertical section through the reactor of the invention;

FIG. 3 is an enlarged scale partial vertical section showing thearrangement of the fuel element assemblies within the reactor of FIG. 1taken along the lines 33 of FIGS. 10 and 11;

FIG. 4 is a greatly enlarged plan section taken along the line 44 ofFIG. 3;

FIG. 5 is a partial enlarged section of a fuel pin and its connection tothe fuel element assembly of FIG. 3;

FIG. 6 is a greatly enlarged one-half plan section of the reactor vesseltaken along the lines 6-6 of FIG. 2;

FIG. 7 is an enlarged plan section of the reactor vessel taken along thelines 77 of FIG. 2;

FIG. 8 is a greatly enlarged partial vertical section of the permanentthermal shield connection to the reactor vessel of FIG. 2;

FIG. 9 is a greatly enlarged quarter view of the bottom face of thelower grid plate assembly;

FIG. 10 is a greatly enlarged quarter view of the upper face of thelower grid plate assembly;

FIG. 11 is a greatly enlarged quarter view of the top face of the uppergrid plate assembly;

FIG. 12 is greatly enlarged partial vertical section of the hold-downspring and piston seal assembly of the upper plenum chamber assembly ofFIG. 2, and

FIG. 13 is a partial vertical section of the lower grid plate assemblytaken along the line 13-13 of FIG. 9.

In FIG. 1 there is shown the nuclear reactor of the present inventionlocated in a heat transfer system for the generation of steam to be usedin the production of electrical power. The system comprises a reactorwherein heat is generated by a controlled chain type fission reactor andhaving bottom mounted control rods 11, a pressurizer 12, a steamgenerator 14 and primary coolant pumps 16. The reactor 10 receivespressurized water at a pressure of 1500 p.s.i. and 481 P. which has beentaken from the steam generator 14 via lines 15, 20 by the primarycoolant pumps 16. The primary coolant liquid flows through the reactorin heat transfer relationship with the fuel contained therein and isheated to 510 F. The heated water then leaves the reactor and flowsthrough the line 22 to the steam generator 14 which is a type asdescribed in the copending application of the common assignee No.428,038 of D. K. Davies et al., filed May 6, 1954, now Patent No.2,904,013 issued September 15, 1959. The primary coolant flows throughthe steam generator 14 in indirect heat exchange relationship with thesecondary coolant to which passage it transfers the heat it received inits passage through the reactor. After its passage through the steamgenerator the secondary coolant exits as saturated steam at 405 p.s.i.The cooled primary coolant then fiows through the line 15 to the coolantpumps 16 to complete the cycle.

The pressurizer 12 is an electrically heated boiler operating at 1500p.s.i. and connected to the primary coolant system by a small linethrough which pressure is transmitted thereto.

Specific details of the theory and construction of the pressurizer 12are set forth in the copending application of Donald F. Judd, Serial No.715,432, filed February 14, 1958, now Patent No. 3,114,414 issued to thecommon assignee. Though the diagram in FIG. 1 sets forth only oneprimary coolant loop it is understood that a number of loops may beconnected to the reactor and that the number of loops in use may bevaried as required, by closing the main stop valves 17, 19 arranged inthe lines 20 and 22.

An important requirement of a nuclear reactor used in the commercialgeneration of electrical power is that the reactor core have an extendedoperating lifetime. An example of a core with an extended operatinglifetime would be one having a lifetime of approximately 600 full powerdays. In such a core it is necessary to provide an excess mass offissionable material in addition to that required to sustain a fissionchain reaction in a cold clean core, i.e., a reactor core in which achain reaction has not been established. The excess mass of fissionablematerial is required to provide reactivity for temperature rise,build-up of neutron absorbing fission products, burnout of fuel andother miscellaneous reaction poisoning effects. To control the excessreactivity present within the reactor core a control system is required.

The control system for the reactor in the present invention comprises aprimary system of bottom mounted control rods 11 supplemented byburnable and soluble poison disposed within the core. The bottom mountedcontrol rods are disopsed within the reactor so that they may beadjustably positioned within the core. Control rod drive mechanisms areprovided to position the control rods, either electro-mechanical orhydraulic systems, or the combination of both, may be used as the drivemechanisms, examples of which are illustrated and described in PatentsNos. 2,735,811, 2,708,656, 2,756,857, and 2,798,847. The control rodmaterial, a substance which will absorb neutrons without reproducingthem, may be selected from a group including hafnium, boron, stainlesssteel or an alloy of cadmium-indium and silver. An automatic controlsystem comprising a pressure sensor, programmer, demand comparator andservo-control actuates the control rod drive mechanism. The control rodsare arranged with a central regulator rod surrounded by a group of shimrods. The shim rods provide coarse adjustments in the reactivity of thereactor while the regulator rod accomplishes rapid, fine adjustment ofthe reactivity.

Burnable poisons are substances with high neutron capture cross sectionwhich have a capture reaction product of low capture cross section andwhich are introduced into a reactor to influence the long-termreactivity variations therein. In the present invention a burnablepoison such as natural boron or europium is mixed with the fuel and actsas a poison to supplement the control rods in holding down excessreactivity. However, due to its nature it is consumed gradually byneutron absorption.

A soluble poison is one which can be introduced into the reactor insolution to absorb neutrons unproductively. In the present invention asupplementary control is obtained by dissolving a soluble poison, forexample boric acid (H in the primary coolant and regulating its strengthby mechanical and chemical means instead of allowing nuclear burnout.Specific details of the theory, operation and contents of the controlsystem are set forth in the copending application of John F. Mumm,Serial No. 721,404, filed March 14, 1958, by the common assignee, nowabandoned.

In FIG. 2 there is shown a preferred embodiment of the nuclear reactor10 used in the commercial generation of steam. The nuclear reactor 10comprises a vertically elongated pressure vessel of circular crosssection which is connected to the primary coolant inlet line 20 at thenozzle 19 and to the outlet coolant line 22 at the nozzle 21. The vesselhas a hemisperically shaped lower end 32, a circular wall 33 and a wallof increased cross section forming an upper flange 34. A plurality ofthreaded studs 38 connect a head member 36 to the flange 34 in pressuretight relationship. Both the pressure vessel and the head member areformed of carbon steel plate with a stainless steel cladding 39 on theinner faces thereof. A vertically extending annular shaped neutronshield tank 40 is formed about the pressure vessel enclosing the centralportion thereof. A body of light water 41 is maintained in the tank 40to provide a lateral neutron shield for the reactor. A layer ofstainless steel wool insulation 42 is contained within a closely fittingcan 44 and arranged about the walls 33 of the pressure vessel to reducethe heat loss from the reactor to the neutron shield tank 40.

The interior of the pressure vessel is generally divided into threezones by transversely arranged upper and lower grid plate assemblies 45,47, respectively. These zones are a lower plenum chamber 46, and anupper plenum. chamber 48, separated by a core chamber 50 which isdisposed between the upper and lower grid plate assemblies.

The reactor internals contained within the pressure vessel 30 aredivided into two classifications, first, permanent reactor internalswhich are integrally attached to the pressure vessel, and second,removable reactor internals.

The permanent reactor internals comprise a vertically disposedfrusto-conically shaped support skirt 52 which provides the main supportfor the removable reactor internals and walls 54, 56 which formvertically extending concentrically arranged and radially spaced openended cylinders which act as permanent thermal shields within the corechamber 50.

The removable reactor internals consist of a lower plenum chamberassembly arranged Within the lower plenum chamber 46, an upper plenumchamber assembly arranged within the upper plenum chamber 48 and a coredisposed within the core chamber 50 between the upper and lower gridplate assemblies 45, 47.

A multiplicity of elongated, longitudinally contiguous fuel elementassemblies 92 of heterogeneous form are geometrically arranged as a coreto undergo a controlled chain type fission reaction. The fuel elementassemblies 92 comprise longitudinally elongated exterior walls 94 joinedtogether to form open ended flow chambers 96 of substantially squarecross section about six inches on a side. The walls 94 of Zircaloy-2, amaterial capable of withstanding the high temperatures generated by thechain type fission reaction and having a low absorption cross sectionfor thermal neutrons. Two adjacent walls 94A, 94B forming the flowchamber 96 are inset for roughly two-thirds of their width extendingfrom a common corner to form one recessed corner 94C which extends alongthe length of the fuel element assembly.

A large number of cylindrically shaped fuel pins 98 are arranged withinthe flow chamber 96 in parallel relationship with the longitudinal axisthereof. The fuel pins .98 comprise an open ended, cylindrically shapedstainless steel tube 100 having an outside diameter of 0.3125 inch and0.02 inch wall thickness. Arranged within the tube 100 is a column ofcylindrically shaped fuel pellets 102 stacked end to end. The fuelpellets 102 are a mixture of fully enriched uranium oxide (U0 and thoria(ThO which has been compressed and sintered to yield a high density andmachined to a close dimensional tolerance. An enriched uranium oxide isone in which the abundance of the U isotope therein is increased abovethe amount it normally contains. A fully enriched uranium oxide is onethat contains more than 90% of the U isotope. The fuel pellets 102 havea diameter of .2675 inch and a length to diameter ratio of approximatelyone. The diameter of the fuel pellets is sufliciently smaller than theinterior diameter of the fuel pin tube 100 to provide an annular space104 between the tube 100 and the fuel pellet 102. An inert insulatingpellet 106 of magnesium oxide or alumina is placed at each end of thecolumn of fuel pellets. End plugs 108 are welded into the open ends ofthe fuel pin tubes 100 to form seals. The insulating pellet 106 servesto reduce the temperature gradient between the fuel pellets and the endplug, thereby reducing the stresses caused by differential expansion.The distance between the interior faces of the end plugs 108 within thetube 100 is greater than the height of the column of pellets 102, 106 toallow for the assembly and differential expansion of the pellets. A heattransfer medium, such as helium, lead, or sodium, is placed within thetube 100 to fill the voids and to reduce the temperature gradient acrossthe voids caused by the spaces. The end plugs 108 secured within theends of the tubes 100' have extensions 109 of reduced circular crosssection.

A number of pairs of spaced parallel tube sheets 112A, 1128 are arrangedwithin the walls 94 to receive and secure therebetween the fuel pins 98by engaging the extensions 109 of the end plugs 108 within openings inthe tube sheets. (See FIGS. 3 and 5.) The fuel pins 98 are arrangedwithin the tube sheets 112A, 112B in a square lattice with a pitch of0.3805 inch. Openings are located in the tube sheets between theconnections of the fuel pins 98 to permit the flow of a coolanttherethrough. The

tube sheets and fuel pins cooperate to form fuel pin bundles having across section which substantially fills the cross sections of the flowchamber 96 within the walls 94. The fuel pin bundles are 16.5 incheslong and have an active fuel region of 15 inches centered longitudinallyin each pin 98. Six bundles are stacked end to end forming the fuelregion of the fuel element assembly 92.

Each fuel element assembly has in inlet nozzle extension adapter 116secured within one end of the flow chamber walls 94 (FIG. 3). Theadapter has an inlet channel 116A of circular cross section arranged tobe engaged in the lower grid plate assembly and a transition section116B varying from circular to square cross section arranged within theflow chamber walls 94. The circular cross section portion 116A extendslongitudinally and co-axially from the flow chamber walls 94.

The upper end of each assembly 92 has an outlet nozzle extension adapter118 secured within the upper end of the flow chamber walls 94. Theoutlet nozzle extension adapter 118 comprises an interior transitionsleeve 119, a spring 120 and an exterior nozzle extension 121. Theexterior nozzle extension 121 has a circular cross section and isremovably attached to the flow chamber walls by means of a bayonet lock122 and extends longitudinally and co-axially from the flow chamberwalls 94. The interior transition sleeve 119 is resiliently held inplace and at one end is slidably engaged within the exterior nozzleextension 121 and on the other end bears on the tube sheet 112B of thefuel pin bundles. The interior transition sleeve 119 has a transitionportion 119A varying from a square to a circular cross section and anoutlet channel portion 119B of circular cross section. The spring isarranged about the outlet channel portion 11913 of the interiortransition sleeve 119 to maintain the fuel pin bundles in position andagainst the exterior nozzle extension 121 on the other end to springload the bayonet lock. The exterior nozzle extension 121 is engagedwithin the upper grid plate assembly and is also resiliently maintainedin position by the spring 120. The outlet nozzle extension adapter 118is arranged so that it may be easily removed for remote refueling of thefuel element assemblies. Additionally the spring loaded outlet nozzleextension adapter 118 provides a hold down against the hydraulic forcesof the flowing coolant and allows for differential expansion bet-weenthe fuel pin bundles and the flow chamber walls 94.

Control rod and fuel element assembly 124 (see circled portion of FIG.6) are arranged in a symmetrical pattern to form the reactor core whichhas the generally cylindrical shape of the reactor pressure vessel wall33. A groupling of four longitudinally contiguous identical fuel elementassemblies 92 are arranged to form each control rod-fuel elementassembly set 124. The four fuel element assemblies are spaced in asquare arrangement with the recessed corner 94C of each fuel elementassembly disposed in the center of the set to form a cross-shapedcontrol rod channel 126 extending throughout the length of the set. Thisassembly pattern is repeated throughout the core (FIG. 6) plus someextra fuel element assemblies disposed about the periphery of the coreto fill out and conform to the circular shape of the reactor vessel.

As shown in FIG. 6 the preferred core has one hundred twenty fuelelement assemblies 92, having fissionable material and twenty-eightdummy element assemblies 93 (shown dotted) having no fissionablematerial therein. The dummy fuel element assemblies 93 have the sameoutside dimensions as the live fuel element assemblies but are arrangedto control the flow of coolant in the core.

The core is arranged so that the number of live fuel element assembliesmay be increased from one hundred twenty to one hundred forty-eightshould this prove necessary or desirable in the future.

The core fuel arrangement is best shown in the one-half section of FIG.6 where the half not shown is exactly as the half shown. Therein thereactor 10 has two fuel concentration zones. Zone A is symmetricallyarranged in the center of the core and contains thirty-two fuel elementassemblies (marked with an A). Zone B is symmetrically disposed aboutzone A and contains eighty-eight fuel element assemblies (marked B). Thefuel element assemblies within zone A contain a fuel having a mixture of5.3 weight percent of fully enriched U0 and the rest ThO while zone Bfuel element assemblies contain a fuel mixture of 6.6 weight percentfully enriched U0 with the remainder ThO The purpose of the two fuelconcentration zone arrangement is to flatten flux gradient across thereactor and thereby provide a more uniform distribution of power in thecore.

Control rods 128 of cross-shaped cross section are movably positionedwithin the core in the control rod channels 126 formed by the controlrod-fuel element assembly sets '124. The control rods 128 are formed offour equal width arms disposed at right angles to each other. Theoverall width of each control rod is 7.5 inches with a thickness of 7inch and an active length of approximately 8 feet. The control rods asset forth above are formed of materials having a high neutron absorptioncross section. The core contains a total of twenty-one control rods, oneregulator rod disposed in the center of the core and twenty shim controlrods disposed symmetrically thereabout. The control rod channels formedbetween the dummy assemblies 93 have placed therein stainless steeldummy rod 127 of substantially the same geometrical cross-section as thecontrol rods to allow correct proportioning of coolant flow through thecore. The control rods 128 are positioned within the core by means of acontrol rod drive mechanism (not shown) external of the reactor 10. Acontrol rod drive shaft 129 (see FIG. 2) passes through each control rodnozzle extension 130 in the base 32 of the reactor vessel and isconnected to a follower rod 131 which is in turn connected to thecontrol rod 128. The follower rod 131 has substantially the samephysical arrangement as the control rod 128 and is made of Zircaloy-2, alow neutron absorption material. As the control rod 128 is withdrawnfrom the core it travels upwardly into the upper plenum chamber 48 andits position within the core is taken by the follower rod 131. Thefollower rod 131 is provided to prevent the formation of large coolantflow spaces through the core when the control rods 128 are withdrawn.The control rod drive mechanism, not shown, is a fail safe type so thatif there is a failure of the control rod drive mechanism system thecontrol rod 128 will fall into the core and scram or shut down thereactor.

Vertically extending stainless steel walls which form an open endedright circular cylinder are disposed laterally about the core providinga removable thermal shield 132, as shown in FIGS. 2 and 7. The removablethermal shield 132 extends above and below a pair of spaced horizontalplanes which define the upper and lower limits of the active fuel regionof the core. Within the removable thermal shield 132 a core shroud 134is arranged about the core. The core shroud 134 is formed of verticallyarranged flat plate joined to form a continuous wall disposed about theouter periphery of the fuel element assemblies 92, 93. The core shroud134 and the removable thermal shield 132 cooperate to form verticallyextending flow passages 136 about the outer boundary of the fuel elementassemblies 92, 93. Horizontally disposed core shroud baffie plates 138,see FIG. 6, having openings 139 therethrough are arranged between thecore shroud 134 and the removable thermal shield 132 to limit the flowof primary coolant through the flow passages 136.

The permanent thermal shields 54, 56 (see FIGS. 2 and 6) are locatedexteriorly of the removable thermal shield 132. The permanent thermalshields 54, 56 extend downwardly from a horizontal plane passing throughthe lower face of the upper grid plate assembly 45 with the innerpermanent thermal shield 54 terminating at a point roughly in ahorizontal plane passing through the bottom limit of the active fuelregion within the fuel element assemblies. The outer permanent thermalshield 56 extends below the inner thermal shield and terminatesapproximately in a horizontal plane through the upper portion of thesupport skirt 52.

Referring to FIG. 8 a permanent thermal shield support assembly 57 isintegrally attached to the pressure vessel wall. The permanent thermalshield support assembly 57 comprises an annular ring '53 attached to theinner surface of the pressure vessel wall 33 and clad with stainlesssteel. A hanger support 59 formed as an open ended cylinder ofrelatively short height is welded to and extends downwardly from theannular ring 58. A circular permanent thermal shield support ring 60 iswelded to the hanger support and in turn provides the immediate supportfor the permanent thermal shields 54, 56 which are welded thereto. Flowpassages 60A, 60B, 60C having adjustable permanent shield orifice plugs61A, 61B, 61C arranged therein are provided through the permanentthermal shield support ring 6%? to control the amount of coolant flowpassing therethrough. A vertically arranged tube 63 passes through andis supported by the ring 58, this tube will be used to containinstrumentation or material that is to be tested by exposure toradiationv An intermediate support 64 is formed on the pressure vesselwall 33 to support the tube 63. Set screw plugs 65A, 65B are adjustablyarranged in the thermal shields 54, 56. Bearing pads 65C are located onthe end of the set screw plugs 65A. The plugs 65A, 65B and pads 65C arefor use in assembly only and do not provide structural support.

The support skirt 52 (see FIG. 2) is arranged in the upper portion ofthe lower plenum chamber 46 and comprises a number of support skirt lugs52A, a support skirt baflle plate 528 and a support skirt bearing ring52C. The support skirt lugs 52A are intermittently arnanged in ahorizontal plane and weldably attached to the pressure vessel wall 33.The intermittent spacing of the support skirt lugs 52A provides openingsfor coolant to pass from the lower plenum chamber to the spaces betweenthe permanent and removable thermal shields 54, 56, 132.

The support skirt baffle plate 528 is formed by a frustoconically shapedwall converging upwardly and welded to the support skirt lugs 52A aroundits lower end. The support skirt bearing ring 52C is horizontallyarranged around and welded to the upper edge of the support skirt baffie plate 5213. The support skirt bearing ring 52C provides thesupport for the lower plenum chamber assembly which in turn supports theother removable reactor internals.

Referring to FIG. 2 the lower plenum chamber assembly is comprised of alower grid plate assembly 47, an inlet flow baffie 76, an inlet flowbaffle diaphragm plate 84 and a number of lower plenum chamber controlrod guide tubes 85. The inlet flow baffle 76 is formed by a downwardlyconverging frusto-conically shaped wall. An inlet flow bafile supportring 78 is arranged about and integrally attached to the upper edge ofthe wall forming the inlet fiow baflle 76 and is bolted to the lowergrid plate assembly 47. The inlet flow baffle 76 has openings 76Atherein to permit the flow of coolant therethrough and to insure uniformflow distribution. The inlet flow baflie diaphragm plate 84 is disposedhorizontally across and attached to the bottom edge of the inlet flowbaffle 76 forming a bottom closure. Openings 84A are arranged in thelower plenum chamber diaphragm plate 84 to permit control rod driveshafts 129 to pass therethrough.

Referring to FIGS. 2, 3, 9, 10 and 13, the lower grid plate assembly 47is horizontally and co-axially arranged within the pressure vessel andis supported on the support skirt bearing ring 52C. The lower grid plateassembly 47 is composed of a lower grid plate 67, a lower grid platehoneycomb support element 68, a number of fuel element assembly inletsleeves 71 and control rod guide tube sleeves 72, an open ended lowergrid plate cylinder 73 and a lower grid plate bearing ring 74. Thehorizontally arranged bearing ring 74 is supported on the support skirt52 and in turn supports the vertically upstanding open ended lower gridplate cylinder 73 which provides the peripheral container for the lowergrid plate assembly. The lower grid plate 67 is horizontally disposedacross the upper edge of the cylinder 73 and provides a wall between thelower plenum chamber 46 and the core chamber 50. The lower grid platehas openings 75A, 75B therein through which fuel element assemblies 92,dummy fuel element assemblies 93, and control rods 128 pass. Locatingholes 75C are arranged in the lower grid plate 67 to position and securethe fuel element assembly inlet sleeves 71. Bolt holes 75D are providedin the lower grid plate 67 to attach the control rod guide tube sleeves72 to the lower grid plate. The lower grid plate honeycomb supportelement 68 is constructed of a network of upstanding perpendicularlyarranged and integrally attached plates disposed within and extendingacross the cross-sectional area of the cylinder 73. They honeycombsupport element as is disposed normal to and below the lower grid plate67 and has its upper edges in contact with the lower grid plate toprovide support therefor. The honeycomb support element divides thelower grid plate 67 into sections having a horizontal cross sectionsubstantially equivalent to the cross section of the control rod-fuelelement assembly sets 124. Variable orifices 70 are located through thelower grid plate at the intersection of the perpendicularly arrangedhoneycomb support element plate. Stifiener plates 69 are situated at theintersections of the lower edges of the plates forming the honeycombsupport element 68 and attached thereto to maintain alignment andspacing. Vertically arranged fuel element assembly inlet sleeves 71which receive the fuel element assembly inlet nozzle extension adapters116 are attached within the openings in the lower grid plate 67.Vertically extending straps 75 are integrally connected to the fuelelement assembly inlet sleeves 71 and to the vertical faces of thehoneycomb support element plates 68 to provide alignment and rigidityfor the sleeves.

A number of lower plenum chamber control rod guide tubes 85 of circularcross section are vertically disposed in the lower plenum chamberco-axial with control rod channels 126 in the core. The lower ends ofthe lower plenum chamber control rod guide tubes 85 re attached to thelower plenum chamber diaphragm plate 84 and the upper ends are connectedto the control rod guide tube sleeves 72 which are bolted to the lowergrid plate 67. The control rod guide tube sleeves 72 are formed bylongitudinally flattening a cylinder having substantially the same crosssection as the lower plenum chamber con trol rod guide tubes 85 to asection of roughly square cross section and having slightly concavesides. The control rod guide tube sleeves 72 and the lower plenumchamber control rod guide tubes 85 provide lateral support for thefollower rod 131 and shaft 129 and also present a sealed flow chamberthrough the lower plenum chamber from the diaphragm plate 84 to thelower grid plate 67. A control rod nozzle seal 87 is disposed betweenthe lower plenum chamber diaphragm plate 84 and the end 32 of thereactor pressure vessel and joins the control rod nozzle extensions 130and the lower plenum chamber control rod guide tubes. An orifice 88 ispositioned in the control rod nozzle seal 87 to admit coolant to theinterior of the control rod guide tubes 85. At the opposite ends of thecontrol rod nozzle extensions a buffer seal 89 is provided to minimizecoolant leakage along the control rod drive shaft 129.

The upper plenum chamber assembly as shown in FIGS. 2, 3 and 11 iscomprised of an upper grid plateassembly 45, a transistion section 155,an upper flow bafile 166, a number of upper plenum chamber control rodguide tubes 172, and a hold down spring and piston seal assembly 180.The upper grid plate assembly 45 is arranged concentrically within theremovable thermal shield 132 providing an outlet annular space 143therebetween for the flow of primary coolant from the core chamber 50 tothe upper plenum chamber-48. The upper grid plate assembly 45 isarranged to receive the outlet nozzle extension adapters 118 of the fuelelement assemblies 92 and to be supported by the fuel elementassemblies. Referring particularly to FIGS. 3 and 11 the upper gridplate assembly 45 comprises an upper grid plate 144, an open ended uppergrid plate cylinder 146, an upper grid plate honeycomb support element148, a number of outlet fuel element assembly sleeves 150 and an uppergrid plate bearing ring 152. The upper grid plate 144 is horizontallyarranged within the reactor pressure vessel and has circular openings144B therein to receive and be supported by the outlet nozzle extensionadapters 118 of the fuel element assemblies 92. The upper grid plate 144provides a wall between the core chamber 50 and the upper plenum chamber48. The upper grid plate 144 also has openings 144A to permit thecontrol rods to pass vertically therethrough from the core into thetransistion section 155 in addition to the openings 144B. The openings144A, 144B through the upper grid plate have chamfers 145A, 145B on thecore chamber side of the upper grid plate to permit the fuel elementassemblies 92 and the control rods 128 to be aligned within the plateopenings 144A, 144B during assembly. These chamfered openings 144A, 144Bwill prevent any damage to the structure of the fuel element assemblies92 or control rods 128 caused by their being out of line while theremovable reactor internals are being assembled. Variable orifices 144Care located in the upper grid plate 144 and are similar to those in thelower grid plate 67 as shown in FIG. 13. Holes 144D are provided in theplate 144 to receive dummy control rods.

Vertically arranged outlet fuel element assembly sleeves 150 areinserted within the upper end plate openings 144B and fixed at aposition above the chamber 145B to receive the exterior nozzleextensions 121 of the fuel element assembly outlet nozzle adapters 118-.Straps 151 are integrally connected to the fuel element assembly inletsleeves and to the vertical faces of the honeycomb support elementplates 148 to provide alignment and rigidity for the sleeves 150.

The open ended vertically extending upper grid plate cylinder 146 issupported about and. welded to the periphery of the upper horizontalface of the upper grid plate 144 to provide a peripheral container forthe upper grid plate assembly 142. The upper grid plate honeycombsupport element 148 is constructed of a network of verticallyupstanding, perpendicularly arranged and integrally attached platesdisposed within and extending across the cross-sectional area of thecylinder 146. The honeycomb support element 148 is disposed normal toand above the upper grid plate to provide support therefor. Thehoneycomb support element divides the upper grid plate into sectionshaving a horizontal cross section substantially equivalent to the crosssection of the control rod-fuel element assembly sets 124. Stilfenerplates 153 are attached to the upper edges of the plates forming thehoneycomb support element 148 at their intersections to maintainalignment and spacing. The upper grid plate bearing ring 152 issupported on and integrally attached to the upper grid plate cylinder146 and has an outside diameter equal to that of the cylinder. Thebearing ring 152 extends inwardly from the cylinder 146 and suppliesstructural support to the honeycomb support element 148 by providing abearing surface about the periphery of the cylinder 146 for the upperedge faces of the plates which form the honeycomb support element 148.

The transition section 155 is supported by the upper grid plate assembly45 and is formed by an open ended transistion section cylinder 156 whichhas an outside diameter and wall thickness substantially equal to thatof the upper grid plate cylinder 146, and an upper and a lower supportring 157, 158 disposed respectively about the upper and lower horizontaledge of and extending inwardly from the outer surface of the transitioncylinder 156. Openings 156A are disposed through the transition sectioncylinder 156 to permit flow of coolant there through. A transitionsection diaphragm plate 159 having a number of openings 159A ishorizontally arranged across the upper end of the transition sectioncylinder 156 to provide a closed end therefor. The upper plenum chambercontrol rod guide tubes 172 are attached to the upper grid plate andextend vertically upward through the transition section 155 to thetransition section diaphragm plate 159 and are secured within itsopenings 159A. The control rod guide tubes 172 are of circular crosssection and have a cap plate 173 disposed across the diaphragm plate endof the tubes. An orifice 174 is provided in the cap plate 173 to permitcoolant flow to exit from the upper plenum chamber control rod guidetubes.

Upper flow bafile 166 is formed by an upwardly diverging wall offrusto-conical shape. Top and bottom upper bafile support rings 167, 168are horizontally disposed, respectively, about the upper and lowerhorizontal edges of the upper flow baffle 166 walls. The upperflowbaflle 166 has at its lower edge substantially the same outside diameteras the transition section cylinder. The upper flow baffle 166 issupported by the transition section 155 through the bottom upper bafflesupport ring 168 which bears on the upper transition section supportring 157. A hold down ring assembly 176 is positioned in the upperregion of the upper plenum chamber 48 and is disposed within thepressure vessel joint. The pressure vessel closure head 36 rests on theupper face of the hold down ring assembly 176. The hold down ringassembly 176 in turn rests on the top upper baffle support ring 167 andcooperates to maintain the removable reactor internals in position.Openings 166A are located through the upper fiow baffle 166 to permitprimary coolant fiow therethrough.

Referring to FIGS. 2 and 12, a hold down spring and piston seal assembly180 is attached to the outer surface of the transistion section cylinder156 to provide a seal between the upper plenum chamber 4-8 and thatportion of the core chamber 56 arranged between the outer surface of theremovable thermal shield 132 and the inner face of the reactor pressurevessel wall 33 within the core chamber 50. The hold down spring andpiston seal assembly 180 comprises a hold down spring bracket 182, ahold down spring 184, a spring bearing block 185, an orifice seal ring186, and a piston seal assembly 192. The hold down spring bracket 182 isweldably attached to the outer surface of the transition sectioncylinder 156 and the hold down spring 164 is mounted therein. The holddown spring 184 acts between the bracket 182 and the bearing block 185;the bearing block acts downwardly on the orifice seal ring 186 which isconcentrically arranged about the bottom of the transition sectioncylinder 156. An orifice seal ring annular space 187 is disposed betweenthe orifice seal ring 186 and the transition section cylinder 156 tolimit the flow therethrough from the core chamber 513 which flow passesbetween the upper grid plate cylinder 1.46 and the inner face of theremovable thermal shield 132. The orifice seal ring 186 bears on thepiston seal assembly 192 whtich in turn is supported on the upper edgeof the removable thermal shield 132 and on the permanent thermal shieldsupport ring 60 and cooperates with the support ring to form a circularseal between the core chamber 50 and the upper plenum 48 chamber. Thepiston seal assembly consists of an annular shaped seal block 193,resilient means 194 to retain the seal block in position, a piston ring195 and a vertically arranged cylindrically shaped cooperating surface196 which combines with the piston ring to provide a seal. Whenhydraulic lift forces and ditferential expansion place the hold downspring 184 in compression, the orifice seal ring 186 maintains theannular space 187 formed about the transition section cylinder 156, andthe piston seal assembly 192 resiliently maintains the seals between thecore chamber 51) and the upper plenum chamber 48. A number of dowels 197are positioned in the transition section cylinder 156 to assist in theremoval of the orifice seal ring 186 on reactor disassembly. Similarly,a number of pins 198 are positioned in the removal thermal shield 132 toaid in the removal of the piston seal assembly 192 on reactordisassembly.

Although the invention has been described as to the physical shape andarrangement of parts, its operation can best be understood by thefollowing description of its operation. Under full load operatingconditions, 126,000 g.p.m. of light water as a primary coolant at apressure of 1500 p.s.i.a. is circulated through the reactor pressurevessel 1t). Approximately 85% of the total flow passes in heat transferrelationship with the fuel element assemblies to remove the heat offission and the remaining is diverted past the control rods 128, theexterior of the fuel element assemblies and about the thermal shields54, 56, 132. There is a flow of the primary coolant throughout theentire volume of the reactor pressure vessel.

Primary coolant is delivered to the lower plenum chamber 46 in thereactor pressure vessel through the inlet connection 19 located throughthe reactor pressure vessel walls 33. The fiow distribution of theprimary coolant is directed by means of the inlet flow baffle 76 throughthe lower plenum chamber 43 for its passage to the core chamber 56.There are several passageways the coolant may follow as it coursesthrough the core chamber 511.

The greater part of the primary coolant passes through the openings 76Ain the inlet flow bafile 76 and flows to the lower grid plate 67. Aboutof the total primary coolant flow passes through the inlet fuel elementassembly sleeves 71 in the lower grid plate 67 to the flow chambers 96within the fuel element assemblies 92 with another 13% flowing aroundthe assemblies. This 13% of the primary coolant enters the core throughthe orifices 76 in the lower grid plate 67, and through the control rodguide tubes 85. The coolant which enters the flow chamber of the fuelelement assemblies 92 flows therethrough in heat transfer relationshipwith the fuel pins 98. The coolant within the fiow chamber passesthrough the openings 113 in fuel pin tube sheets 112A, 112B andcirculates about the individual fuel pins 98. After the coolant exitsfrom the flow chamber 96 through the outlet nozzle extension adapter 118it enters the interior of the transition section cylinder 156 within theupper plenum chamber. The openings 156A provided in the transitionsection permit the primary coolant to flow to the outlet connection 21arranged in the upper plenum chamber 48.

In addition to the primary coolant which enters the core through theorifices '70 in the lower grid plate 67, an additional quantity of thecoolant enters the core chamber 50 through the lower plenum chambercontrol rod guide tubes 85. Coolant is admitted to the control rod guidetubes 85 through the orifices 88 in the control rod nozzle seals 87. Thecoolant entering the core through the lower grid plate orifices 70 andthrough the control rod guide tubes 85, circulates through the spacesbetween the fuel element assemblies 92 and the control rods 128 and alsopasses about the periphery of the core and flow upwardly between theremovable thermal shield 132 and the core shroud 134 passing through thecore shroud baflle plates 138. This portion of the coolant leaves thecore either through the upper plenum chamber control rod guide tubes 172or through the annular space about the core. The coolant flowing throughthe upper plenum chamber control rod guide tubes 172 passes through theorifices 174 in the control rod guide tube cap plates 173 located in thetransition section diaphragm plate 159 and then flows through theopenings 166A in the upper flow baffle 166 to the outlet connection 21.The coolant which passes through the annular space enters the upperplenum chamber through the orifice seal ring 186 and then flows to theoutlet connection 21.

The remaining coolant, which amounts to roughly 2%, flows through thatportion of the core chamber between the outer face of the removablethermal shield 132 and the reactor pressure vessel wall 33. The coolantenters this space through the open spaces provided between theintermittently spaced support skirt lugs 52A. The coolant fiows upwardlyabout the permanent thermal shields 54, 56 and the outer face of theremovable thermal shield 132. Orifices 61A, 61B, 61C are in thepermanent thermal shield support rings 60 to pass the coolant from thethree flow channels formed between the concentrically arranged removableand permanent thermal shields 132, 54, 56 and the interior surface ofthe pressure vessel Wall 33 within the core chamber 50 to the upperplenum chamber 48. This portion of the flow then passes to the outletconnections 21.

To further illustrate the preferred embodiment of the invention, thebelow Table I gives the details of the reactor.

TAB LE I Reactor Date:

Reactor Type. Internal Thorium Converter. Neutron Energy Thermal.

Steam Condition From Reno 405 psig; 449 F.

Primary Coolant Pressurized; Light Water.

Temperature, F 481 in; 510 out. Flow, g.p.m 126,000.

Pressure drop, p.s.1

41 core; 116 tot Pressures, p.s.i.a

al 1,800 design; 1,500 operating.

Core Size 6.5' ft. Diameter by 8.0 ft. High.

B.t.11. 1 hr. ft 112,000.

Tho -U mixture. Stainless Steel.

304 Stainless Steel. Inoonel-X. Zircaloy2.

0.2675 in. nominal.

Fuel Pellet diameter Cladding outside diameter 0.3125 in. nominal.Cladding thickness 0.020 in. nominal. Fuel pin spacing (square lattice)-0.3805 in. nominal. Pins per bundle 206.

Bundles per fuel element Fuel Elements in core Fuel Element WallThicknes Heat transfer area Metal to water ratio CoreVolume Percentages:

0.180 in. nominal. 15,170 sq. ft. 1.122.

Water (Moderator and Coolant)--. 48.

Zircaloy 11.

ThOzUO 27.

Control Rods 2.

Stainless Steel (Cladding and tube 12.

sheets).

Although the present invention has been described using light water asthe coolant moderator, it is contemplated that the apparatus andarrangements may be effectively used when employing other primarycoolants, such as heavy water, organic liquids, and liquid metals.

The preferred embodiment illustrated utilizes a mixture of highlyenriched uranium oxide and thoria; however, it is contemplated that theinvention will effectively operate using mixtures of fissionablematerial, such as Uranium 233 and Plutonium 239 with fertile materialsuch as Uranium 238 and natural uranium. Moreover, the invention shouldnot be limited to oxides of the fissionable and fertile material but itcontemplates the metallic form of these materials either as pins orplates. Additionally, the invention would be equally effective where thefissionable material would take the form of mixtures of the variousknown fissionable materials, such as 2% of U and 3.3% of U The controlrod-fuel element assembly sets 124 have been illustrated as containingfour assemblies placed around a centrally located control rod 128, butit is contemplated that the invention would be equally effective if moreor less than this number of assemblies were located around a centrallylocated rod. For instance, the fuel element assemblies may take theshape of triangles and be arranged generally in a pentagon shape so thatthere would be five assemblies arranged about a crosscontrol rod havingfive blades thereon.

The materials of the fuel element pin tube or cladding and of the walls94 forming the fiow chamber of the assembly 92 may be constructed of anyof the relatively low thermal neutron cross-section materials presentlyknown, such as the various Zircaloys 2, 3, 4 or zirconium, aluminum, andmagnesium.

While in accordance with the provisions of the statutes, I haveillustrated and described herein a specific form of the invention nowknown to me, those skilled in the art will understand that changes maybe made in the form of the apparatus disclosed without departing fromthe spirit of the invention covered by my claims, and that certainfeatures of the invention may sometimes be used to advantage without acorresponding use of the other features.

What is claimed is:

1. In a nuclear reactor a supercritical mass of fuel containing amixture of enriched uranium and additional fertile material, saidmixture arranged in a core to undergo a controlled chain type fissionreaction, the invention comprising a plurality of uniformly constructedelongated and longitudinally contiguous fuel element assemblies ofheterogeneous form, each of said assemblies containing said mixture ofenriched uranium and fertile material and with the enriched uraniumtherein being of substantially uniform enrichment throughout said core,said fuel element assemblies within said core divided into a number ofsymmetrically arranged longitudinally extending fuel concentrationzones, each of said zones having a uniform fissionable to fertilematerial weight percent ratio in the fuel element assemblies positionedthere in, whereas fuel element assemblies positioned in different zoneshave a substantially different fissionable to fertile material weightpercent ratio and contributing a substantial portion of the powerinitially developed in the core.

2. In a nuclear reactor a supercritical mass of fuel containing amixture of enriched uranium and additional fertile material, saidmixture arranged in a core to undergo a controlled chain type fissionreaction, the invention comprising a plurality of uniformly constructedelongated and longitudinally contiguous fuel element assemblies ofheterogeneous form, each of said assemblies containing said mixture ofenriched uranium and fertile material and with the enriched uraniumtherein being of substantially uniform enrichment throughout said core,said fuel element assemblies within said core divided into a centrallyarranged inner fuel concentration zone and at least one annular shapedouter fuel concentration zone disposed concentrically about said innerzone, each of said inner and outer zones having a uniform fissionable tofertile material weight percent ratio in the fuel element assembliespositioned therein, whereas fuel element assemblies positioned indifferent zones have a substantially different fissionable to fertilematerial weight percent ratio with the ratio being lowest in the innerzone and increasing outwardly with each zone, and each zone contributinga substantial portion of the power initially developed in the core.

3. In a nuclear reactor a supercritical mass of fuel containing amixture of uranium oxide (U0 which is enriched in the fissionableisotope (U and thorium oxide (Th0 said mixture arranged in a core tounder go a controlled chain type fission reaction, the inventioncomprising a plurality of uniformly constructed elongated andlongitudinally contiguous fuel element assemblies of heterogeneous form,each of said assemblies containing said mixture of enriched uraniumoxide and thorium oxide and having the uranium oxide of substantiallyuniform enrichment throughout the core, said fuel element assemblieswithin said core divided into a centrally arranged inner fuelconcentration zone and at least one annular shaped outer fuelconcentration zone disposed concentrically about said inner zone, eachof said inner and outer zones having a uniform enriched uranium oxide tothorium oxide weight percent ratio in the fuel element assembliespositioned therein, whereas fuel element assemblies positioned indifferent zones have a substantially different enriched uranium oxide tothorium oxide weight percent ratio with the ratio being lowest in theinner zone and increasing outwardly with each zone, and each zonecontributing a substantial portion of the power initially developed inthe core.

4. In a nuclear reactor a supercritical mass of fuel 15 containing amixture of enriched uranium and additional fertile material, saidmixture arranged in a core to undergo a controlled chain type fissionreaction, the invention comprising a plurality of uniformly constructedelongated and longitudinally contiguous fuel element assemblies ofheterogeneous form, each of said assemblies containing said mixture ofenriched uranium and fertile material, said fuel element assemblieswithin said core divided into a number of symmetrically arrangedlongitudinally extending fuel concentration zones, and each of saidzones having a uniform fissionable to fertile material weight percentratio in the fuel element assemblies positioned therein, whereas fuelelement assemblies positioned in different Zones have a substantiallydifferent fissionable to fertile material weight percent ratio and eachZone contributing a substantial portion of the power initially developedin the core.

References Cited by the Examiner REUBEN EPSTEIN, Primary Examiner.

CARL D. QUARFORTH, LEON D. ROSDOL,

- Examiners.

1. IN A NUCLEAR REACTOR A SUPERCRITICAL MASS OF FUEL CONTAINING AMIXTURE OF ENRICHED URANIUM AND ADDITIONAL FERTILE MATERIAL, SAIDMIXTURE ARRANGED IN A CORE TO UNDERGO A CONTROLLED CHAIN TYPE FISSIONREACTION, THE INVENTION COMPRISING A PLURALITY OF UNIFORMLY CONSTRUCTEDELONGATED AND LONGITUNDINALLY CONTINGUOUS FUEL ELEMENT ASSEMBLIES OFHETEROGENEOUS FORM, EACH OF SAID ASSEMBLIES CONTAINING SAID MIXTURE OFENRICHED URANIUM AND FERTILE MATERIAL AND WITH THE ENRICHED URANIUMTHEREIN BEING OF SUBSTANTIALLY UNIFORM ENRICHMENT THROUGHTOUT SAID CORE,SAID FUEL CONCENTRATION ZONES, EACH OF SAID ZONES HAVING A UNIFORMFISSIONABLE TO FERTILE MATERIAL WEIGHT PERCENT RATIO IN THE FUEL ELEMENTASSEMBLIES POSITIONED IN DIFFERENT ZONES HAVE A SUBSTANTIALLY DIFFERENTFISSIONABLE TO FERTILE MATERIAL WEIGHT PERCENT RATIO AND CONTRIBUTING ASUBSTANTIAL PORTION OF THE POWER INTIALLY DEVELOPED IN THE CORE.